This invention relates to communications systems and methods, and more particularly to systems and methods for communicating messages among devices that are serially connected.
Wireless and wired communications systems and methods are widely used for communicating among devices. The devices may include computers or communication devices, such as cellular radiotelephone base stations. Communication between a central unit and a plurality of devices may be arranged in a xe2x80x9chub and spokexe2x80x9d or xe2x80x9cstarxe2x80x9d arrangement, in which a central unit is individually connected to a plurality of devices. FIG. 1 illustrates such a hub and spoke arrangement for a cellular radiotelephone communications system.
FIG. 1 illustrates a conventional terrestrial cellular radiotelephone communication system 20. The cellular radiotelephone system may include one or more radiotelephones 21, communicating with a plurality of cells 36 served by base stations 23 and a Mobile Telephone Switching Office (MTSO) 25. Although only three cells 36 are shown in FIG. 1, a typical cellular network may comprise hundreds of cells, may include more than one MTSO, and may serve thousands of radiotelephones.
The cells 36 generally serve as nodes in the communication system 20, from which links are established between radiotelephones 21 and the MTSO 25, by way of the base stations 23 serving the cells 36. Each cell will have allocated to it one or more dedicated control channels and one or more traffic channels. The control channel is a dedicated channel used for transmitting cell identification and paging information. The traffic channels carry the voice and data information. Through the communication system 20, a duplex radio communication link 32 may be effected between two mobile stations 21 or between a radiotelephone 21 and a landline telephone user 33. The function of the base station 23 is commonly to handle the radio communication between the cell and the mobile station 21. In this capacity, the base station 23 functions chiefly as a relay station for data and voice signals. As shown, a plurality of base stations 23 are directly connected to the MTSO 25 in a hub and spoke arrangement.
Traditional analog radiotelephone systems generally employ a system referred to as Frequency Division Multiple Access (FDMA) to create communications channels. As a practical matter well-known to those skilled in the art, radiotelephone communications signals, being modulated waveforms, typically are communicated over predetermined frequency bands in a spectrum of carrier frequencies. These discrete frequency bands serve as channels over which cellular radiotelephones communicate with a cell, through the base station or satellite serving the cell. In the United States, for example, Federal authorities have allocated to cellular communications a block of the UHF frequency spectrum further subdivided into pairs of narrow frequency bands, a system designated EIA-553 or IS-19B. Channel pairing results from the frequency duplex arrangement wherein the transmit and receive frequencies in each pair are offset by 45 Mhz. At present there are 832, 30-Khz wide, radio channels allocated to cellular mobile communications in the United States.
Conventional cellular systems also may employ frequency reuse to increase potential channel capacity in each cell and increase spectral efficiency. Frequency reuse involves allocating frequency bands to each cell, with cells employing the same frequencies geographically separated to allow radiotelephones in different cells to simultaneously use the same frequency without interfering with each other. By so doing, many thousands of subscribers may be served by a system of only several hundred frequency bands.
Another technique which may further increase channel capacity and spectral efficiency is Time Division Multiple Access (TDMA). A TDMA system may be implemented by subdividing the frequency bands employed in conventional FDMA systems into sequential time slots, as illustrated in FIG. 2. Although communication on frequency bands f1-fm typically occur on a common TDMA frame 210 that includes a plurality of time slots t1-tn, as shown, communications on each frequency band may occur according to a unique TDMA frame, with time slots unique to that band. Examples of systems employing TDMA are the dual analog/digital IS-54B standard employed in the United States, in which each of the original frequency bands of EIA-553 is subdivided into 3 time slots, the digital IS-136 standard employed in the United States and the European GSM standard, which divides each of its frequency bands into 8 time slots. In these TDMA systems, each user communicates with the base station using bursts of digital data transmitted during the user""s assigned time slots. A channel in a TDMA system typically includes one or more time slots on one or more frequency bands. See, for example, U.S. Pat. No. 5,850,292 to Wang et al., that is assigned to the assignee of the present invention, the disclosure of which is hereby incorporated herein by reference.
Another hub and spoke arrangement of communication devices is shown for an indoor cellular communication system in U.S. Pat. No. 5,887,267 to coinventor Fugaro, that is assigned to the assignee of the present invention, the disclosure of which is hereby incorporated herein by reference. FIG. 3 herein corresponds to FIG. 1 of the ""267 patent. As described therein, capacity limitations of cellular radiotelephone systems have been addressed by using microcells or picocells, that is, low power cellular transmissions that provide coverage over a smaller area. The smaller microcells can allow more cells to exist within a predefined geographic area, thereby allowing an increase in the number of users that can be serviced within that geographic area. A particular application of microcell technology is indoor cellular radiotelephone services.
As illustrated in FIG. 3, a conventional indoor cellular network 320 includes one or more mobile stations or units 322, one or more base stations 324 also referred to as Radio Heads (RH), a Radio Control Interface (RCI) 326 also referred to as a Control and Radio Interface (CRI), and a mobile switching center (MSC) 328. Although only one cell 330 is shown in FIG. 3, a typical indoor cellular network may have several cells 330, each cell usually being serviced by one or more base stations 324. The number of base stations 324 may depend on the channel capacity of the cell 330. Each base station typically supports anywhere from 4-12 channels, depending upon its site. The cell 330 typically has one or more control channels and one or more voice/data (hereafter referred to as xe2x80x9ctrafficxe2x80x9d) channels allocated to it. The control channel typically is a dedicated channel used for transmitting cell identification and paging information.
Each base station 324 is connected to the radio control interface 326 by a radio interface link 332 in a hub and spoke configuration. The radio control interface 326 exchanges signals between the base stations 324 and the mobile switching center 328. Specifically, the radio control interface 326 converts the traffic and control information from the format received over the radio interface links 332 into a format suitable for transmission over a dedicated transmission link 334 interconnecting the radio control interface (RCI) 326 to the MSC 328. In the reverse direction, the RCI 326 converts the traffic and control information received over transmission link 334 into a format suitable for transmission over radio interface links 332 to the respective base stations 324.
The MSC 328 is the central coordinating element of the overall cellular network 320. It typically includes a cellular processor 336 and a cellular switch 338, and provides an interface to the public switched telephone network (PSTN) 340. Through the cellular network 320, a duplex radio communication link 342 may be effected between two mobile units 322 and a landline telephone user 344. The function of the base stations 324 is commonly to handle the radio communications with the mobile units 322. In this capacity, the base stations 324 also supervise the quality of the link 342 and monitor the received signal strength from the mobile units 322.
Another conventional interconnection of a central unit and a plurality of devices is a serial connection, also referred to as a xe2x80x9ccascadexe2x80x9d connection or a xe2x80x9cdaisy chainxe2x80x9d connection. In this type of connection, a plurality of devices are serially connected such that a preceding device is connected to a succeeding device. Downlink messages are communicated from the central unit to the plurality of devices that are cascaded from the central unit. Uplink messages are communicated from the plurality of devices that are cascaded from the central unit, to the central unit. As was the case in hub and spoke connections, the messages may be TDMA frames having a plurality of slots. One well-known example of a cascade connection is the Small Computer Systems Interface (SCSI) that is used to connect peripherals to a personal computer.
Cascading may be advantageous because separate connections from each device to the central unit may be eliminated. The wiring of the communication network thereby may be simplified. Addition of devices also may be simplified. Finally, when each connection from a central unit is a leased line, such as a T1 line, the cost of the communication network may be reduced, since the number of leased lines from the central unit may be reduced.
Cascaded connections may be particularly advantageous in a microcell/picocell system such as an indoor cellular communications system of FIG. 3, wherein the Radio Control Interface (RCI) 326 may be located in one building, whereas the plurality of base stations 324 may be located in a second building. Thus, a leased line, such as a T1 line, may be used to connect the RCI 326 to a first base station, and the remaining base stations can be cascaded so that additional leased lines need not be used.
Unfortunately, it may be difficult to address cascaded devices efficiently, to allow unique identification of each cascaded device for uplink and downlink messages. In particular, when respective downlink and uplink messages are communicated from and to a central unit, to and from a plurality of devices that are cascaded from the central unit, each of the devices may need a unique address so that downlink and uplink communications may be properly directed to and received from an intended device. Accordingly, when installing a device, the device may need to be configured with a specific address. Moreover, a consistent addressing scheme may need to be maintained as devices are added or removed, so that addresses of other cascaded devices may need to be changed when devices are added or removed.
More specifically, when devices are cascaded in a TDMA communications system, each device may need to know the TDMA time slot mapping scheme in order to communicate with the central unit. Unique device addresses may need to be assigned and each device may need to be configured locally to enable communications with the central unit. This may reduce the flexibility and may increase the cost of systems and methods that communicate messages among a plurality of cascaded devices.
It is therefore an object of the present invention to provide improved systems and methods for communicating messages among a plurality of cascaded devices.
It is another object of the present invention to provide systems and methods for communicating messages in TDMA frames among a plurality of cascaded devices.
It is still another object of the present invention to provide systems and methods for communicating messages among cascaded devices, wherein each device need not be configured with a unique address to enable communication.
These and other objects may be provided, according to the present invention, by communicating messages among a plurality of devices that are serially connected, such that a preceding device is connected to a succeeding device, by receiving a message from a preceding device, bit shifting the message that was received from the preceding device and transmitting the bit shifted message that was received from the preceding device to a succeeding device. When a message is received from a succeeding device, it also is bit shifted and the bit shifted message that was received from the succeeding device is transmitted to the preceding device. Preferably, messages that are received from the preceding device are shifted in a first direction such as left by a predetermined number of bits and messages that are received from the succeeding device are shifted in a second direction that is opposite the first direction such as right by the predetermined number of bits. Preferably, the predetermined number of bits corresponds to at least one TDMA slot.
Prior to shifting the message that was received from the preceding device left by the predetermined number of bits, the predetermined number of leftmost bits is extracted from the message that was received from the preceding device. Moreover, after shifting the message that was received from the succeeding device right by the predetermined number of bits, the predetermined number bits is inserted into the leftmost part of the message that was received from the succeeding device.
Accordingly, each of the cascaded devices preferably extracts a downlink message from at least the first TDMA slot and then shifts the downlink message to the left by at least one slot. Similarly, each of the cascaded devices shifts an uplink TDMA message to the right by at least one slot and inserts its message into at least the first slot.
Since each of the cascaded devices inserts a message into and extracts a message from the same slot(s), it need not be locally configured with a predefined address. Only the central unit need have knowledge of the order in which devices are cascaded, to thereby place the messages in and retrieve the messages from the appropriate TDMA slots. The devices themselves may be identical and do not need to include a programmed address. The installation and configuration of cascaded devices thereby may be simplified and the implementation of addressing in the devices also may be simplified. It will be understood that the present invention may be provided as communication methods, communication systems and individual devices that are cascaded.